1. Field of the Invention
The present invention relates to the structure of ink jet head and, more particularly, to the structure of ink jet head of a type using an electro-thermal transducer. The present invention provides a cheap and highly reliable head.
2. Related Background Art
In recent years use of metal substrate is under research as a substrate for ink jet heads of the type to eject ink by use of the electro-thermal transducer. This is from the following reason. The silicon substrate has been used heretofore as the substrate for the ink jet heads of this type. However, when the print width of ink jet head is intended to be set wide, the silicon substrate has a limit of increase in the size of substrate, because it is normally provided in the form of wafer. Therefore, desires exist for a substrate material excellent in workability. The ceramic substrate, which has been used in the conventional thermal heads and the like, is also one of substrate materials excellent in workability, but it is normally made in the structure provided with a glass-coat-baked layer (glaze layer) on a partial or entire surface because of its poor smoothness of substrate surface. In the case of the ink jet head of the type using the electro-thermal transducer, however, the head experiences such driving that the heater portion of electro-thermal transducer reaches temperatures near 1000.degree. C. within a very short period, and it is thus difficult to apply the ceramic substrate to the ink jet head of the type using the electro-thermal transducer in terms of durability and heat-radiating characteristics of the glaze layer.
On the other hand, when the metal substrate is used as a substrate of ink jet head, surface roughness on the substrate also affects the durability of heat-generating resistor provided on the substrate, and thus metals the surface roughness of which can be readily decreased are used. Metals excellent in machinability, including Ni, SUS, and Al, have been used as such metals.
However, when Ni or SUS is used, the substrate thereof exhibits poor thermal characteristics (heat conductivity) and radiation of heat is not sufficient in applications of wide print width of ink jet head as described above. Therefore, such substrates are not suitable for high-speed printing.
Therefore, Al has been used heretofore as a material of the metal substrate. For example, Japanese Patent Application Laid-open No. 9-1806 discloses such structure that a heat-accumulating layer is provided through Cr on the metal substrate of Al in order to enhance adhesion between the metal substrate and the heat-accumulating layer. However, Al is poor in chemical stability and might be corroded by chemicals used during fabrication of head or components in the ink. The structure described in the above application resists corrosion from the surface of substrate, but in the structure Al is not protected by Cr at the end face of substrate where discharge openings are provided, and Al will be thus corroded by the ink intruding from the portion of discharge opening to the end face of substrate, which Could result in partial peeling of the Cr layer and the heat-accumulating layer. If the corrosion causes an impurity to be mixed in the ink, this impurity will form burnt deposits (scorches) on the heat-generating portion.